LTC1760
34
1760fc
For more information
www.linear.com/LTC1760
Figure 6. PowerPath Driver Equivalent Circuit
–
+
+
–
GB1I
GB1O
Q8
Q7
FROM
BATTERY
1
BAT1
SCP
25mV
20mV
OFF
OFF
1760 F06
EAP
CP
SWP
TO LOAD
C
L
R
SC
OPERATION
8.1 Autonomous PowerPath Switching
The LOPWR comparator monitors the voltage at the
load through the resistor divider from pin SCN. If LTC
(POWER_OFF) is low and the LOPWR comparator trips,
then all of the switches are turned on (3-Diode mode)
by the Autonomous PowerPath Controller to ensure that
the system is powered from the source with the highest
voltage. The Autonomous PowerPath Controller waits
approximately 1 second, to allow power to stabilize, and
then reverts back to the PowerPath switch configuration
requested by the PowerPath Management Algorithm. A
power fail counter is incremented to indicate that a failure
has occurred. If the power fail counter equals a value of 3,
then the the Autonomous PowerPath Controller sets the
switches to 3-Diode mode and BatterySystem-
StateCont(POWER_NOT_GOOD) will be set, provided
the LOPWR comparator is still detecting a low power
event. This is a three-strikes-and-you’re-out process
which is intended to debounce the POWER_NOT_GOOD
indicator. The power fail counter is reset when battery or
AC presence change.
8.2 Short-Circuit Protection
Short-circuit protection operates in both a current mode
and a voltage mode. If the voltage between SCP and
SCN exceeds the short-circuit comparator threshold
V
TSC
for more than 15ms, then all of the PowerPath
switches are turned off and BatterySystemState-Cont
(POWER_NOT_GOOD) is set. Similarly, if the voltage
at SCN falls below 3V for more than 15ms, then all of
the PowerPath switches are turned off and POWER_
NOT_GOOD is set high. The POWER_NOT_GOOD bit is
reset by removing all power sources and allowing the
voltage at V
PLUS
to fall below the UVLO threshold. If
the POWER_NOT_GOOD bit is set, charging is disabled
until V
PLUS
exceeds the UVLO threshold and the Charger
Algorithm allows charging to resume.
When a hard short-circuit occurs, it might pull all of the
power sources down to near 0V potentials. The capacitors
on V
CC
and V
PLUS
must be large enough to keep the circuit
operating correctly during the 15ms short-circuit event.
The charger will stop within a few microseconds, leaving
a small current which must be provided by the capacitor
on V
PLUS
. The recommended minimum values (1µF on
V
PLUS
and 2µF on V
CC
, including tolerances) should keep
the LTC1760 operating above the UVLO trip voltage long
enough to perform the short-circuit function when the
input voltages are greater than 8V. Increasing the capaci-
tor across V
CC
to 4.7µF will allow operation down to the
recommended 6V minimum.
8.3 Emergency Turn-Off
All of the PowerPath switches can be forced off by set-
ting the DCDIV pin to a voltage between 8V and 10V. This
will have the same effect as a short-circuit event. DCDIV
must be less than 5V and V
PLUS
must decrease below
the UVLO threshold to re-enable the PowerPath switches.
The LTC1760 can recover from this condition without
removing power. Contact Applications Engineering for
more information.
8.4 Power-Up Strategy
All three PowerPath switches are turned on after V
PLUS
exceeds the UVLO threshold for more than 250ms. This
delay is to prevent oscillation from a turn-on transient
near the UVLO threshold.
